Carbon nanotubes are, in general, elongated tubular bodies which are typically only a few atoms in circumference. They are hollow and have a linear fullerene structure. The length of the carbon nanotubes potentially may be millions of times greater than their molecular-sized diameter.
Carbon nanotubes are thus sheet(s) rolled to form a long tubular fibers of nanodimensions. Depending upon the number of sheets contained within the tube, the carbon nanotube can be described as single walled (SWCNT) (one sheet) or double walled (DWCNT) (two sheets) or multiwall (MWCNT) (3 sheets or more). The average diameter of SWCNTs range from 0.5 nm to up to 5 nm, with the majority being in the 0.5 to 2.0 nm range. The SWCNT may have metallic or semiconducting electrical properties depending upon the registration at the seam where the 2 sides of the sheet meets to form the tube. With DWCNTs having a tube within a tube construction, the diameter is primarily in the 1.5 to 5 nm range. MWCNTs have been further segmented into few walled varieties (3 to less than about 10 sheets) with diameters primarily in the 5 to 20 nm range, and true multiwall consisting of 10 to <30 sheets and with diameters greater than 100 nm possible. Depending upon the technique used to manufacturer the particular CNT, the lengths can range from less than 1 micron to 10 tens of microns and possibly even millimeters.
Since the first report of carbon nanotubes by Iijima in 1991 (Nature 354 (1991) 56-58), CNTs have been one of the most researched materials due to their unique properties. The rolled sheet provides them with high elasticity and tensile strength, good electrical and thermal conductivities, good thermal stability and chemical resistance. For example, it has been estimated that SWCNTs conduct heat and electricity better than copper or gold, and have 100 times the tensile strength of steel but at only a sixth of the weight. Extraordinarily small sizes can be made. For example, carbon nanotubes are being produced that are approximately 1/50,000th the width of a human hair.
As a result of these properties, CNTs are well suited for a variety of applications utilizing their electrical properties or mechanical properties or even their ultra small tubular structure either individually or in combination. For example, researchers have demonstrated that CNTs can provide the same level of conductivity to insulating plastics as afforded by electroconductive carbon black but at much lower loadings and in many cases, with the added benefit of improved mechanical properties. The electrical conductivity, in particular of SWCNTs, is such that their potential usage within transparent conductive electrodes has been of particular interest. Therefore, applications ranging from electromagnetic shielding to radar absorption to electrically conducting plastics and coatings are possible. The thermal conductivity properties of CNTs coupled with their nano-size has made them interesting candidates for thermal management in next generation microelectronic devices. In the area of mechanical reinforcement, many researchers have demonstrated matrix improvements upon the addition of CNTs to form ultra-strong and light weight composites. The high surface area and charge carrying ability of CNTs also makes them ideal candidates for energy storage devices such as batteries and capacitors. CNTs have even been used to help probe other nanomaterials by using them as ultrafine tips in atomic force microscopy (AFM).
There are four common methods for producing carbon nanotubes, namely: 1) laser vaporization techniques; 2) electric arc techniques; 3) gas phase techniques; and 4) chemical vapor deposition. In laser vaporization and electric arc techniques, the CNTs are produced by vaporizing graphite, with or without metal catalyst present, using either a laser beam or an electric arc, respectively. The development of catalyst systems to allow the controlled growth of CNT type is an area of continued focus. In gas phase techniques, a carbon source is run across a bead of catalyst particles under pressure and heat to produce normally a continuous stream of CNTs. A well known gas phase Process is the HiPco process developed by Richard Smalley which utilizes carbon monoxide as the carbon source and has been shown to be adept at producing large quantities of SWCNTs.
A central difficulty in working with and incorporating CNTs into materials and devices is that the tubular sheet responsible for their remarkable properties also renders them nearly insoluble in solvents. Therefore, substantial research effort has been expended towards the efficient dispersion of CNTs. Methods of rendering nanotubes soluble can be grouped into two broad categories: (1) covalent modification of the nanotube cylinder with groups that improve the interaction with the solvent and (2) treatment of the nanotube with a non-covalently bonded dispersion agent. Perhaps the simplest, though versatile, covalent modification strategies are the oxidation and fluorination of nanotubes. Subsequent reaction of the oxidized or fluoro groups has allowed the nanotubes to be further derivatized using a variety of agents. In addition, standard small molecule reactions such as diazonium, cycloaddition, carbene, radical, and carbanion chemistries have proven applicable towards nanotube modification. While covalent modification results in nanotubes that are soluble in solvents not compatible with pristine nanotubes and a reduction in their aggregation behavior, chemical functionalization unfortunately results in the incorporation of defects into the aromatic system responsible for their unique properties. To circumvent the necessity of chemically modifying the nanotube to affect solubility, compounds such as surfactants, polyaromatics, biopolymers, synthetic polymers, and encapsulation agents have been explored to disperse CNTs. For instance, carbon nanotubes have been solubilized in organic solvents and water by polymer wrapping, but a disadvantage of this approach is that the polymer is very inefficient in wrapping the small-diameter single-walled carbon nanotubes produced by the HiPco process (the only high purity material currently produced on a large scale) because of high strain conformation required for the polymer.
The dispersing agent approach does not have the same net loss of some of the CNTs unique properties. A vast number of the dispersants which have been proposed are mere encapsulates/wetting agents and therefore do not provide for the interfacial adhesion needed in mechanical applications. Some of the most effective dispersant systems are those that incorporate π-π interactions, such as conducting polymers, highly aromatic polymers, and polyaromatic groups. In the case of the conjugated/aromatic polymers, a difficulty is that the chemistry used to make such systems involves multi-step synthesis including metal coupling steps, and therefore the dispersants are expensive. Also, the dispersant is useful with only one type of CNT or the dispersant is only effective in one or a few solvent systems, and therefore lacks general applicability in most cases. For example, while sodium dodecylsulfate has been widely used to disperse SWCNTs in aqueous solution, the ionic nature of sodium dodecylsulfate requires a limited pH range for effectiveness and the dispersant is limited to aqueous environments. Polymeric dispersants also have solvent limitations such as with poly(4-vinylpyridine) which can disperse CNTs in alcohols but not water (Chemistry of Materials 2004, 16, 3940-3910). Therefore, the development of a dispersant platform technology allowing SWCNTs, DWCNTs, and MWCNTs to be effectively dispersed in a wide-range of matrices is of importance.
The carbon nanotube literature does contain a few references to phthalocyanines being tried to disperse CNTs. For example,                X. Wang et al. Q. Mater. Chem., 2002, 12, pp 1636-1639) report functionalizing carbon nanotubes by non-covalent absorption of tetra-tert-butylphthalocyanine from chloroform solution, but do not address the stability of the resulting composite nor its ability to be dispersed in a solvent or other matrix.        Chemistry of Materials 2004, 16, 3940-3910 illustrates that polymeric dispersants also have solvent limitations, indicating that poly(4-vinylpyridine) can disperse CNTs in alcohols but not in water.        Y. Wang et al. (Materials Science and Engineering B 117, 2005, 296-301) formed “composites” of multiwall carbon nanotubes and copper phthalocyanine following covalent attachment to both of a long allyl chain. The authors report “enhanced solubility” of the composite in organic solvents, and UV-vis spectra show strong electronic interaction between the two “composite” constituents. However, when MWCNT's and copper phthalocyanine were separately incorporated in a polymer matrix (polyvinyl butyral), UV-vis spectra showed absence of significant electronic interaction between the two, and poor dispersibility. It should also be noted that covalently attaching chemical groups to carbon nanotubes is, as earlier explained, undesirable.        Ma et al. (Journal of Cluster Science, 2006, 17, 599-608) disclose the absorption of copper and copper-free 2,9,16,23-tetra-tert-butyl-29H,31H-phthalocyanine onto single walled carbon nanotubes. While limited functionalization of the nanotubes is reported, nothing is disclosed about their dispersibility. Moreover, the three solvents utilized in the study (dimethylformamide, chloroform, and 1,2-dichlorobenzene) are all known to allow some amount of SWCNT solubility (Chem. Comm. 2001, 193-194). Thus, these experiments do not demonstrate that the phthalocyanines utilized are in fact dispersing the SWCNTs.        US Patent application 2007/0137701 (WO 2004/060988 A3) discloses the uses of a phthalocyanine pigment in the form of Solsperse RTM 5000® to disperse carbon nanotubes in xylene. However, the data presented in Table 1 of the disclosure indicates that the degree of dispersion achieved was very poor and that the dispersion was not stable (Table 1, XF001), and in fact, only occurred when a secondary synergistic polymer was added (Table 1, dispersion XF003-XF017).        Hatton et al. (Langmuir, 2007, 23, 6424-6430) studied the interaction of surface oxidized multiwall carbon nanotubes (o-MWCNTs) and tetrasulfonate copper phthalocyanine (TS-CuPc). They found that dispersions of o-MWCNTs in aqueous solutions of TS-CuPc were stable toward nanotube flocculation and exhibit spontaneous nanostructuring upon spin casting onto a uniform film. The film was composed of an o-MWCNT “scaffold” decorated with phthalocyanine molecules self assembled into extended aggregates. The ionic nature of sodium dodecylsulfate requires a limited pH range for effectiveness and the dispersant is limited to aqueous environments. There is no suggestion that the “scaffolding” has an ability to disperse in any solvent or other organic media. Furthermore as in the case of X. Wang, the nanotubes used have been chemically modified.        Thus there exists, in the literature, no systematic protocol for generating dispersants which, preferably by non-covalent interaction with carbon nanotubes, are capable of producing stable nanotube dispersions in organic or other media. Such stable dispersions would be of great utility in permitting exploitation of the unique properties of nanotube discussed earlier. Therefore, the development of a dispersant platform technology allowing SWCNTs, DWCNTs, and MWCNTs to be effectively dispersed in a wide-range of matrices is of significant technical and commercial importance.        